1. Field of the Invention
The present invention relates to a piezoelectric element including a piezoelectric body layer which is deformed in accordance with a voltage applied thereto, a liquid droplet ejection head including plural the piezoelectric elements, and a liquid droplet ejection apparatus including the liquid droplet ejection head.
2. Description of the Related Art
Conventionally, liquid droplet ejection apparatuses, such as ink jet printers, comprise a liquid droplet ejection head for ejecting ink liquid droplets. This liquid droplet ejection head comprises plural liquid droplet ejection ports, plural pressure generating chambers each connected to the respective liquid droplet ejection ports, and a piezoelectric element actuator for each pressure generating chamber; generates a pressure wave (an acoustic wave) to the liquid filled up in the pressure generating chamber, using the piezoelectric element actuator; and by that pressure wave, ejects ink liquid droplets from the liquid droplet ejection port connected to the pressure generating chamber. Generally, the piezoelectric element actuator is configured to comprise a piezoelectric element which is deformed in accordance with a voltage applied thereto, and a diaphragm which dilates and contracts the pressure generating chamber by the deformation of the piezoelectric element.
Conventionally, such a piezoelectric element is constructed by film formation of a piezoelectric body layer, and an electrode layer on the front and back surfaces of the piezoelectric body layer in sequence, and as examples of materials, gold/nickel/chromium for the first electrode layer, a piezoelectric material for the piezoelectric body layer, and chromium/gold for the second electrode layer can be mentioned.
When a voltage is applied to the respective piezoelectric elements in the liquid droplet ejection head thus configured as above for driving them to cause the respective liquid droplet ejection ports to eject ink liquid droplets, there arises a difference in the polarization condition among the piezoelectric elements due to the difference in the driving history among them, resulting in the amount of deformation displacement (the piezoelectric characteristic) varying even if an electric field of a given potential difference is applied, and a variation in the amount of ink liquid droplet which is ejected from a particular liquid droplet ejection port may be caused.
Then, in Laid-Open Publication 2003-80698, the art for canceling the polarization by applying a voltage reverse to that applied to the piezoelectric element when ink liquid droplets are ejected is disclosed. With this art, the variation in the amount of ink liquid droplet ejected from a particular liquid droplet ejection port can be suppressed.
However, although, with the art as disclosed in Laid-Open Publication 2003-80698, the variation in the amount of ink liquid droplet ejected can be suppressed, there has been a problem that the longer the period of time of driving the liquid droplet ejection head, the heavier the degradation of the piezoelectric characteristic of the piezoelectric element itself will be. If the degradation of the piezoelectric element is progressed, the amount of displacement when the electric field is applied is reduced, which may make the ejection of ink liquid droplets impossible.